THE FUNCTIONS OF THE ORGANISM 7 



after undergoing appropriate and usually very 

 complex changes within the organism, having for 

 their purpose the alteration of these substances into 

 " food," are built up into the mechanism and become 

 more or less permanent parts of it, or are employed 

 in other ways, which at present need be referred to 

 only in very general terms. If properly nourished 

 the organism is able to perform work, not merely 

 visible work, but also internal work not necessarily 

 apparent to the senses ; it repairs waste in its various 

 parts, in many cases adds new parts or increases 

 the size and complexity of parts already in existence 

 a series of phenomena commonly united under the 

 term growth and also lays aside surplus material over 

 immediate needs in appropriate forms in some part 

 of its body for use on a subsequent occasion. 



During the entire course of its life-history the 

 organism is exposed to an ever changing " environ- 

 ment," a term used to indicate everything, living or 

 non-living, palpable or impalpable, outside it. The 

 stimuli exerted by the environment may be in some 

 cases injurious, but in other cases they are distinctly 

 advantageous ; these stimuli are constantly varying sensi- 

 in character, in time of application, and in intensity, tivity. 

 Manifestly, it must be of the utmost importance to 

 the organism to be capable not only of appreciating 

 these impulses, but also of responding to them in 

 such a way as to protect itself from such as are hurtful, 

 and to take every advantage of such as are favourable 

 to its well-being. The most superficial observation, 

 in fact, teaches us that the organism is sensitive to 

 stimuli and capable of responding to them by move- 

 ment, structural adaptation and so on. It must be 

 noted, however, that the possession of sensitivity 

 does not necessarily involve that power of apprecia- 



